The objective of this project is to determine the relation of blood levels of serine protease inhibitors (antithrombin III) and platelet microaggregates to thromboembolic disease. Non-invasive methods of detection of arterial and venous thrombosis using ultrasonic Doppler techniques will be evaluated for their usefulness in small vessel thrombosis. The subject model to be used will primarily be the sick term and preterm infant who has an increased risk for thromboembolic complications. Methods of prevention and treatment (heparin, exchange transfusion, dextran, aspirin, dipyridamole) will also be prospectively evaluated.